Well, Shangpa Kagyu is a completely different lineage than any of the "greater" or "lesser" schools/lineages stemming from Marpa, through Mila and Gampopa.

"Kagyu" means, simply, "oral lineage." Or maybe "Word lineage."

In any case, the Shangpa Kagyu transmissions are alive and well, and actually are practiced in Karma Kagyu, Sakya, Gelukpa, Jonangpa, even by some Nyingma Lamas. Kyabje Dilgo Khyentse Rinpoche practiced Shangpa and even wrote a Monlam for the flourishing of the lineage.

But as an institution, it didn't last long after Khedrup Khyungpo Naljor's immediate disciples. There have been recent attempts to "re-institutionalize" it, but by and large it flourishes mainly amongst the students of the previous Kyabje Kalu Rinpoche, who dubbed his transmission the DakShang Kagyu, to symbolize both Dakpo (Gampopa) and Shangpa lineages.

"Absolute Truth is not an object of analytical discourse or great discriminating wisdom,
It is realized through the blessing grace of the Guru and fortunate Karmic potential.
Like this, mistaken ideas of discriminating wisdom are clarified."
- (Kyabje Bokar Rinpoche, from his summary of "The Ocean of Definitive Meaning")

Here is an interview with Gharwang Rinpoche about the Zurmang / Surmang Kagyu lineage. There interview touches on Trunga Rinpoche, Rechung Nyengyu and many wonderful stories about His Holiness the 16th Karmapa. Enjoy.